When comparing Photo Mechanic vs Darktable, the Slant community recommends Darktable for most people. In the question“What are the best Lightroom alternatives?”Darktable is ranked 1st while Photo Mechanic is ranked 4th. The most important reason people chose Darktable is:

Most modules, by default, affect the whole image, but have the option to only be applied to masked-off areas (be those drawn masks, parametric ones, or a combination of the two).

Pro

Allows for local adjustments

Most modules, by default, affect the whole image, but have the option to only be applied to masked-off areas (be those drawn masks, parametric ones, or a combination of the two).

Pro

Good batch editing capabilities

Darktable allows applying the same set of operations to multiple images and saving your history stack as a style (you can pick exactly which modules).

Pro

Customizable

Darktable allows favoriting modules and remapping hotkeys.

Pro

Fast and Flexible

A very well considered UI makes edits fast and fluid. You can redo or undo any step without disturbing any other part of your edit. The UI doesn't get in the way.

Pro

Supports tethered capture

On the camera set it to use USB Remote. Open Darktable, on the left side under Import, click scan for devices. The camera should appear. Click on tethered shoot. Next on the right side look for the gear icon above the battery n/a and click it. go to the session options tab, and change the base directory to the location you want to save photos. close that settings window and try taking a picture. It should come right up.

Pro

Edits are saved to a separate file

Original files are untouched by any edits. No worries on what was done before or if an accidental save occurs. All work is separate from the originals.

Pro

Feature rich

There are a lot of different modules.

Cons

Con

Asset management only

Photo Mechanic is only an asset management program, there's no support for photo editing, etc.

Con

No official Windows version

Whilst official builds exist for Linux and OSX, this isn't the case for Windows (though unofficial versions do exist).

Con

Can be overwhelming

There are a lot of different modules. You're going to need some time watching tutorials to understand how to get the best out of it.

Con

For tethered capture on Linux you may need additional software installed that is not a dependency for Darktable